As described comprehensively in U.S. Pat. No. 4,288,327, the deposition of solids onto heat transfer surfaces of steam generating equipment is a major problem. Common contaminants in boiler feedwater that can form deposits are calcium and magnesium salts (hardness), carbonate salts, sulfite, phosphate, siliceous matter, and iron oxides. Any foreign matter introduced into the boiler in soluble or particulate form will tend to form deposits on the heat transfer surfaces. Formation of deposits on the heat transfer surfaces will decrease the efficacy of the heat transfer and can lead to overheating, circulation restrictions, damage to the system, loss of effectiveness, and increased cost due to cleaning, unscheduled outages, and replacement of equipment. In extreme case, catastrophic tube failure can occur.
Deposit control agents are frequently added to the feedwater of boilers. Their ultimate objective is to inhibit the formation of deposits on the heat transfer surfaces and to facilitate the removal of any deposits in the blowdown. Typically, this is accomplished via two mechanisms: a solubilization mechanism, where chelants or chelant type molecules form soluble complexes with the deposit forming species which are removed in the blowdown; and, an adsorption mechanism, where the deposit control agent adsorbs on the surface of the particulate matter and either inhibits the formation of the deposit, modifies crystal formation, or disperses the deposit that is being formed, and makes it more easily removable.
Phosphates, chelants and polymeric dispersants are frequently used in various combinations in boiler treatment programs. The phosphate is used to modify inorganic salt form and precipitate hardness or iron species; the chelants have the ability to complex and prevent the deposition of many cations under boiler water conditions. In higher pressure boilers, phosphate is also used for pH control and since it maintains the system at a pH where corrosion is minimized, it also acts as a corrosion inhibitor.
Polymers are used to disperse particulate matter, either the precipitates formed with the phosphate treatment, or solid or colloidal matter already present. To some extent polymers can also act as chelants to solubilize cations.
Polymers that have been used in boiler water treatment include naturally occurring and modified natural polymers such as, lignosulfonates and carboxymethyl cellulose. Synthetic anionic polymers are the more preferred materials recently, and include carboxylated polymers, sulfonated polymers and polyphosphoric acids. Copolymers incorporating combinations of the above functionalities are also used. Examples of effective synthetic polymers include polyacrylic or polymethylacrylic acids and copolymers of the two monomers; sulfonated styrene, polymaleic acid or anhydride, copolymers of sulfonated styrene and maleic anhydride, and others.
In the use of polymeric dispersants, the polymers are fed to maintain a bulk concentration which is many times higher than the effective amount of polymer needed for adsorption on the surface of the particulate matter or the heat transfer surfaces, and for chelation of hardness, etc. That is, the concentration of polymer on the surface is not only determined by the affinity of the polymer for the surface, but also by the equilibrium between the adsorbed species and the bulk species. Thus, where a treatment program might utilize 50-100 ppm of the polymeric dispersant, only 1-10 ppm of active species might be necessary if the polymer could more effectively be brought into contact with the surfaces in question. The excess dispersant can itself contribute to the impurities in the boiler and in the steam produced. Dispersants can degrade under boiler conditions, leading to organic materials such as organic acids which can be present in the steam. Thus, steam purity can be adversely affected by such polymeric dispersants. Furthermore, the organic acids can lead to corrosion in the boiler and in the areas contacted by the steam.
In many boiler designs, heat fluxes are not uniform throughout the entire unit due to design miscalculations. It is known that deposit weight densities (a measure of the amount of boiler deposition increase as heat fluxes increase, approximately as the square of the heat flux. This nonuniformity in heat transfer can lead to "hot spots" in a boiler where the heat flux can be as much as 5 times the average heat flux. These hot spots are predisposed to failure. It is often the case that in an effectively treated boiler there will still be many tube failures in the areas of high heat flux.
In commonly assigned application Ser. No. 168,288 an improved boiler water treatment combination of nonionic surfactants and certain polymeric dispersants together with chelants and/or phosphates is disclosed. The combination allows the polymer to be more effectively adsorbed onto the surfaces of particulate matter or heat transfer surfaces in a boiler.
The use of nonionic surfactants in combination with other boiler treatment actives previously cited in a single package is somewhat limited by the water solubility of such surfactants. Such surfactants generally exhibit decreased solubility at higher storage temperatures often encountered and in aqueous solutions having a high salt concentration. The onset of product instability and surfactant separation manifests itself in a cloud point. The cloud point is the temperature above which aqueous solutions become turbid and eventually form two phases. The water solubility of nonionic surfactants is dependent on the hydrophilic characteristics of the ether linkages in the polyoxyethylene chain. These ether linkages are readily hydrated at room temperature. An increase in temperature reduces the forces of hydration and the surfactants become less water soluble. Most dissolved salts, organic and inorganic, have a greater affinity for water than do the ether linkages in the nonionics and dehydrate surfactant.